We study theoretically the effect of an external field on the nematic–smectic- $A$ transition close to the tricritical point, where fluctuation effects govern the qualitative behavior of the transition. An external field suppresses nematic-director fluctuations by making them massive. For a fluctuation-driven first-order transition, we show that an external field can drive the transition second order. In an appropriate liquid-crystal system, we predict the required magnetic field to be of order 10 T. The equivalent electric field is of order $1\mathrm{V}/\mu \mathrm{m}$.

• Received 24 March 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.4796